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and therefore does not lower the specific gravity as much as would 

 be expected, since most determinations of the specific gravity of berg 

 ice give values not far from 0.90. 



SALINITY 



"V\Tien sea water freezes, the first ice crystals that form are practi- 

 cally salt-free, but they are surrounded by a brine that is saltier 

 than the unmodified sea water and tends to sink below the surface. 

 When the crystals cement together, some of this brine is entrapped 

 among them, so that, on the average, a piece of newly formed ice 

 will have an appreciable salt content. A microscopic examination 

 of a section of such ice would show areas of nearly pure frozen water 

 alternating with zones of brine. As the temperature decreases, the 

 freezing point of the brine is reached, more water solidifies, and the 

 remaining brine becomes more concentrated. This process may con- 

 tinue until the brine cells are so concentrated that they become sat- 

 urated with respect to salt. At very low temperatures, crystals of 

 salt will also be found interspersed in the mass of sea ice. 



The faster the ice forms, the greater the salt content, since more of 

 the brine will be trapped in the ice structure without a chance to sink. 

 This fact is illustrated in the following table, from observations by 

 Malmgren : 



SALINITY OF SEA ICE AS A FUNCTION OF TEMPERATURE 



Temperature of formation, °F 3 —18 —22 —40 



Salinity, parts per thousand (%o) 5.64 8.01 8.77 10.16 



The saltiest piece of ice encountered by Malmgren had a salinity 

 of 14.59%o. The salinity of the sea water in which it was formed 

 was around 30%o. Since the rate of freezing determines the salinity 

 of sea ice, and the thicker the ice the lower the rate, the salinity of 

 newly formed ice will decrease from the surface downward. The 

 following table, again from Malmgren, illustrates this : 



SALINITY OF NEW SEA ICE AS A FUNCTION OF DEPTH 



Distance from surface, inches- 2. 4 5. 2 18 32 37 



Salinity (%o) 6.74 5.28 5.31 4.37 3.48 3.17 



A snow covering over the floe will result in the formation of ice 

 of lower salinity, since the rate of freezing, as illustrated in figure 2, 

 is lowered. 



Once formed, however, ice tends to freshen. It is well known that 

 the freezing point of ice decreases as pressure increases ; this is illus- 



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